Microelectronic devices, such as micro-scale electronic, electro-mechanical or optical devices are generally fabricated on and/or in work pieces or substrates, such as silicon wafers. In a typical fabrication process, a wafer is placed in an electrolyte containing metal ions. A blanket layer or patterned layer of metal is plated onto the substrate by conducting electric current through the electrolyte and through a seed layer on the substrate. The substrate is then cleaned, etched and/or annealed in subsequent procedures, to form devices, contacts or conductive lines.
Currently, most micro-electronic devices are made on substrates plated with copper. Although copper has high conductivity, it typically requires a barrier layer such as tantalum nitride (TaN) to prevent diffusion of copper into the substrate. As the features get smaller, the barrier layer required for copper occupies a relatively larger volume, because a minimum barrier layer thickness must be maintained to prevent copper diffusion, regardless of feature size. In addition, consistently obtaining void-free copper filling of ever smaller features is increasingly difficult.
One approach proposed for overcoming these technical challenges is to replace copper with a metal that does not require a barrier layer, such as cobalt. Although cobalt has a higher resistance than copper (6 uOhm-cm for cobalt versus 2 uOhm-cm for copper), cobalt may not require a barrier layer because it does not diffuse into the silicon or dielectric. Using cobalt instead of copper may also help to overcome the increased copper resistivity due to scattering of the charge carrying electrons and the decrease in the cross sections of the conductor lines. However, existing systems and methods for plating cobalt for this purpose are disadvantageous relative to currently available copper processes. Accordingly, improved systems and methods for creating microelectronic devices with cobalt are needed.